Utilities distribution # 1974 Southhampton
For a given system voltage, the power transfer capability of an underground cable circuit is determined by the amount of current which may be passed through the conductor. Although secondary heating effects occur due to dielectric and sheath losses, proximity effects, etc., the primary limitation is that due to the Joule heating effect in the cable conductor, the resultant temperature rise being in turn limited mainly by the long-term ageing characteristics of the insulation, although in very large conductor cables, the effects of thermo-mechanical forces may become important.
Modern 275kV and 400kV underground cable systems employ copper conductors up to 2,600mm2 (4sq.in.) and are insulated with an oil-filled paper tape dielectric for which the maximum operating temperature, based on a 50-year life, is 85OC, although the shortterm excursions. to higher temperatures are permissible under fault and temporary overload conditions. The value of the current in a given cable conductor which will cause this temperature rise is in turn dependent upon the thermal impedance between the conductor and the heat sink. This IS normally the ground surface, and since the cables are usually buried at about a depth of one metre, the major component of this thermal impedance is provided by the soil. One straightforward way, therefore, in which the power transfer capability of an underground cable circuit may be significantly increased is to eliminate this impedance by providing an alternative heat sink at or near the surfaces of the cables.
l'his has been achieved on some 250km of 275kV supergrid cable circuits by installing flexible water pipes alongside the power cables and of similar size, Fig.l(a) separate pipe cooling. A further development, used for very heavy uu~y circuits at 400kV, is to install the cables inside large water pipes, Fig. 1 (b) integral cooling. This development is the subject of a comparison paper ("The use of plastics pipes for an integral cooled 400kV cable system" Barton and Cliffe) and will not be discussed here. In both systems the water is circulated in a closed pipe circuit, with combined pumping and cooling stations at about 1-3km intervals, depending upon the route profile, Fig.2.